Field of the Invention
The present invention relates to a calibration configuration for setting an adjustable impedance.
Calibration of an integrated circuit requires the presence of a precisely defined reference resistor, relative to which the circuit can be calibrated.
To determine the system characteristics of a circuit configuration, it is necessary to specify the output impedance in order to determine reliable values with regard to time behavior of the signal outputting, voltage consumption, and current consumption. Fluctuations with regard to the output impedance cannot be defined exactly on account of process fluctuations, operating temperature of the semiconductor chip, etc. Various calibration methods are employed nowadays in order to set specific electrical quantities to a desired amount and to eliminate manipulated variables that change during operation. A preferred method is, for example, the calibration of a circuit configuration for setting a desired output impedance relative to a reference resistor. Electronik-Grundlagen [Principles of electronics], 9th edition, Verlag Europa-Lehrmittel, Europa-No.: 31789, page 298, shows a circuit configuration corresponding to FIG. 1 having two resistors R1 and R2, which are connected in series in a first current path, and having two resistors R3 and R4, which are connected in series in a second current path. The positive supply potential VDDQ and the negative supply potential VSSQ of a supply voltage are fed to the ends of the current paths. The resistor R3 illustrated in the second current path is a resistor that is to be calibrated and can be set with regard to its resistance or impedance. Between the current paths, which also represent voltage dividers, a voltmeter is located in the actual bridge path. If the voltages U1 and U3 and, respectively, U2 and U4 across the resistors R1, R2, R3, and R4 are of the same magnitude, the null indicator exhibits no deflection. The bridge is balanced. Because the resistors are proportional to the voltages, the variable resistor can be determined by a ratio calculation:U1/U2=U3/U4.
This also leads to the relationship: R1/R2=R3/R4.
Consequently, the resistor R3 can be derived:
 R3=R4×(R1/R2).
The circuit configuration illustrated is also referred to as a resistance measuring bridge for measuring an electrical resistance by current or voltage comparison of the resistor to be measured and of the known resistors.
With the use of a reference resistor for calibrating an output impedance of a circuit configuration, the two configurations described below are predominantly provided in a circuit configuration.
One possibility is, for example, the use of an off-chip reference resistor—not situated within the semiconductor chip—relative to which the circuit configuration is calibrated. Such a method has the advantage that the reference resistor situated externally with respect to the semiconductor chip can be set very precisely so that the circuit configuration can be calibrated very exactly. One disadvantage of this solution, however, is the need for external reference resistors to be provided for each semiconductor chip situated on a circuit board, relative to which reference resistors the individual circuits of a semiconductor chip can be calibrated with different requirements. The configuration of external reference resistors situated outside a semiconductor chip, furthermore, has the disadvantage that the space on the system circuit board is restricted and, consequently, an economic and cost-effective circuit board design can no longer be achieved.
A further possibility is to dispose the reference resistor within the semiconductor chip containing the circuit to be calibrated. Although such a configuration eliminates the problem of an increased space requirement on the circuit board, it, nonetheless, influences the accuracy of the reference resistance during operation because the latter is operated under the same ambient conditions as the semiconductor chip itself and is, thus, subject to the corresponding fluctuations.
The specification and setting of a reference resistor can only be effected in the production process. The reference resistor can be altered and set depending on manufacturing tolerances and device specifications in the manufacturing process by metal options, fuses, or other physical processes.
A circuit configuration that is calibrated relative to a reference resistor may be contained, for example, in an output driver stage of an off-chip driver, the calibration of the circuit configuration effecting a calibration of the output driver stage. Output driver or amplifier stages generally include complementary field-effect transistors. At least one transistor of an n-channel and a p-channel type are present, which transistors are connected in series. A plurality of transistors of the same type may be connected in parallel with the p-channel and n-channel field-effect transistors. The resistor or the impedance is formed by at least one of the field-effect transistors of the output driver, the connection or disconnection of the respective parallel field-effect transistors enabling the desired resistance to be set. The reference resistor and the resistor of the output driver form a voltage divider in a series circuit. A partial voltage to be tapped off between the two resistors is fed to a comparator with another, fixedly defined voltage, which comparator compares the voltages fed to it. The resistor to be set is set by a control signal generated by the comparator until the two voltages fed to the comparator correspond to one another. The voltages may correspond to one another, for example, if the fixedly defined voltage and the partial voltage correspond to half the supply voltage of the voltage divider. The resistances of the two resistors also correspond to one another in this case.
The problem of a reference resistor subject to the operating fluctuations is manifested here: if such a reference resistor deviates from its desired resistance by 10%, then the voltage to be tapped off between the resistors will amount to half the supply voltage only when the resistor to be set likewise has a deviation of 10% of the desired resistance.